Seat valves with a constant opening pattern often participate in proportional load lowering of load forks of forklifts. The seat valves involved may be actuated directly or under pilot control. Use of seat valves is necessary, since forklift manufacturers require a so-called “lift-dense” load retention because only very slight leakage is permissible. Preventing leaks in the hydraulic circuit ensures that the load fork cannot be lowered independently, with or without load, something which would present a safety problem. The function of restricting the maximum volume flow independently of the load is assumed in the conventional solutions by a constant-volume flow regulator connected in series to the other components of the hydraulic circuit.
The control devices used for this purpose presently are disadvantageous. During lowering of the load fork when empty, only the dead weight of the moving parts, especially that of the load fork, is available on the lift mast to send the hydraulic fluid from the operating cylinder (plunger cylinder) back to the tank as a component of the hydraulic circuit. The pressure on the cylinder may drop to values<8 bar in certain lift mast configurations.
The greater the number of components through which the flow must progress during lowering, the lower is the volume flow. The conventional constant-volume flow regulator presents a particularly great obstacle, since the aperture selected for the regulator must be so small that a control pressure difference of at least 7 bar may be established. Lower control pressures would result in unstable behavior in the hydraulic circuit and may also be unacceptable for safety reasons. Another problem is presented by the falling characteristic required by the customer for the volume flow regulator. Consequently, for considerations of safety and practicability, it must be ensured that the lowering speed will decrease with increases in the load. While this is effected by the constant-volume flow control unit in the conventional control devices, in theory this arrangement as well results in increased instability within the hydraulic circuit.
DE 37 08 143 C1 discloses a safety circuit for the control circuit of a power lift. In the circuit, the position of the hydraulic jack is assigned a nominal value as a function of the pressure differential between adjacent control lines. The pressure in a control line is affected by a load receiving element, and the pressure level in the power lines is affected by a nominal value transmitter. A control circuit is provided by which the drive of the working chamber of the power lift is disconnected from the control valve when the pressure medium supply circuit is disconnected. This switching state is maintained after starting of the pressure medium supply circuit until the desired value set at the nominal value transmitter is brought to the actual position value. This arrangement ensures reliable prevention of unintentional raising or lowering of an operating device mounted on a tractor after the tractor has been started, because an adjustment of the nominal value transmitter has inadvertently been made with the pressure medium supply circuit disconnected. Damage and accidents are thereby excluded. In the case of the conventional solution, however, the lowering behavior of the operating device is as unfavorable as in the case of the technical solutions in this field as described.